Interpretive Summary: Brucella abortus is a disease of cattle that causes abortion and associated economic losses in infected herds. The brucellosis infection of bison and elk in Yellowstone National Park and surrounding areas may pose a risk for transmission to cattle. Studies have found that bison bulls composed 45 to 52% of bison that leave the park during winter months with approximately 52% of these bulls testing seropositive for brucellosis. It is likely that any management plan to reduce or eliminate brucellosis in Yellowstone National Park bison would need to address the prevalence of the disease in male bison. In this study, we evaluated the biosafety of the Brucella abortus strain RB51 vaccine in adult male bison. Although the vaccine strain persisted in a similar manner to that previously reported for bison heifer calves, no adverse clinical signs or pathologic lesions were noted in adult bison bulls. Transient shedding of strain RB51 in the semen was noted in some bison. Our data suggest that strain RB51 is safe for use as a vaccine in adult bison bulls. This data will be of benefit to the Department of the Interior, the United States Fish and Wildlife Service, and bison producers in managing their bison herds to prevent or eliminate brucellosis infections. The demonstration that strain RB51 is safe in adult bison bulls also benefits the USDA Animal and Plant Health Inspection Service in eliminating brucellosis in the United States and preventing its reentry from other countries by providing a safe vaccine that can be used in bulls to enhance protection against brucellosis.

Technical Abstract:
Adult bulls compose a high proportion of the bison population in Yellowstone National Park and have a high seroprevalence for brucellosis. In these studies, we characterized clearance of the vaccine strain, antibody responses, potential shedding, and histologic lesions in reproductive tissues following vaccination of adult bison bulls with Brucella abortus strain RB51. In study 1, 12 bison bulls were vaccinated subQ with 1.02x10*10 CFU of strain RB51 and 3 were inoculated subQ with saline solution. Semen and rectal, nasal, and ocular swabs were obtained to evaluate shedding. Four vaccinates and one control were necropsied at 10, 20, and 30 wks after vaccination. In study 2, bison bulls were vaccinated subQ with saline solution, or by hand (2.9x10*10 CFU) or ballistically (2.1x10*9 CFU) with strain RB51. Strain RB51 was recovered at various times from semen of 3 of 12 vaccinated bison in study 1. At necropsy, strain RB51 1was recovered from bison at 10 (4/4) and 20 wks (1/4), but not at 30 wks (0/4) after vaccination. In study 2, strain RB51 was recovered from lymphoid tissues of hand- (2/15) and ballistically-vaccinated (3/15) at necropsy, 13 wks after vaccination. Strain RB51 was not recovered at any time from saline-inoculated bison. In both studies, histologic lesions in testes, epididymis, and seminal vesicles were minimal and did not differ between strain RB51-vaccinated and saline-inoculated bison. Data from these studies indicate that strain RB51 does not induce significant inflammatory lesions in reproductive tissues of adult bison bulls. Clearance of strain RB51 is similar to that reported for bison calves. The significance of the transient shedding of strain RB51 in the semen of some vaccinates is unknown.